I'm with you. I had a ZRX1200R, great bike. I can tell you the second hour on the hwy is no where near as much fun as the first. It's like water torture, a few drops is ok, after 300 it's a bit tedious.
You could, given that you replace the stock seat . But it certainly isn't a touring bike.
Mine is an 04 XB9S.
I did a trip to Deals Gap this summer, on the stock seat and with the stock flyscreen. 1300 miles in 3 days. Almost all of it was backroads (Blue Ridge Parkway, for example). My only complaint was the stock seat. Since I'm only 5-08, I can lean forward a bit and find a sweet spot where the wind just goes over my helmet. Wind blast was only a problem on the short highway stretches when getting up around 80 mph.
After getting the Streetfighter seat, did a 320 mile Sunday ride (all backroads) and could have done another 200 miles when I came back. The seat really makes a difference.
Fuel capacity is marginal, but I got great mileage when riding slower. On the Parkway, I probably averaged 55 mph and better than 65 mpg.
Also, if you ride aggressively the stock Dunlops on the non-CityX Lightnings wear out fast. Many guys are only getting 3000 miles or so on them...something to consider before starting a long trip.
REAL ANSWER FROM AN ACTUAL XB12S OWNER: I rode over 1000 miles in 18.5 hours including gas and food to join the "thousand mile club" on my 2004 Black XB12 Lightning and was extremely impressed. Yes it is comfortable enough for 500-600 mile days, Easily. I'd say anything up to 700 without feeling any real fatigue if you're stopping occasionally to enjoy the trip. I would feel it after 700 miles on anything however. Someone above recommended a Ulysses. That's a great suggestion if you're wanting to make a habbit out of long trips. Any trips over 5 days would utilize the extra luggage capability. I managed a 5 day trip with only Buell saddle bags and a tank bag. BTW, the 1000 miles was Key West, FL to Florence, AL if anyone wants to punch it up in yahoo or google maps. Great ride.
I am an older owner of a 2004 XB12S (47 years old) with race kit and Corbin seat. I'm also rather big 300 pounds and 6'2" so this applies to larger riders.
You didn't say how old or tall you were but this is what I have felt on my longer rides.
With the Corbin seats I am good for a 500 mile day or so. After that my legs start getting that cramped in feeling and I feel like I am ready to get off. I would suggest the tank bag that goes over the air box but remeber the more bags and weight you add to this little bike the less responsive it will feel.
You might want to try and get a test ride first if possible to see if the bike will be large enough.
I routinely do 400-500 mile days on my 2003 XB9S (15,000 miles without a hitch). This bike is excellent for sport touring on twisty backroads. IMO, due to the overall compactness, this is not the best choice for long hwy stretches.
I would look to the Ulysses or maybe an FJR1300 if long hwy days are in you plans.
I think its more about you than it is about the bike. If you can do it i bet the bike will. get some helibars and giver hell man. i don't know what buell bags look like but i'm bettin there are cheaper and better alternatives.